The present invention is generally directed to forming various articles from thermoset polymers. More particularly, the present invention is directed to a process of first extruding a thermoset prepolymer composition into a desired shape and then crosslinking the composition to form the various polymeric articles, including films, fibers, filaments, fibrous webs, and the like.
Polymeric articles, such as fibers and films, are useful for a wide variety of applications. For instance, thermoplastic polymeric fibers and films have been used in the past for making fluid absorbent wipers, towels, industrial garments, medical garments, medical drapes, and the like. Such articles are also used in recreational applications, such as for making tents and car covers. Films and nonwoven fabrics made from polymeric fibers have also achieved especially widespread use in the manufacture of personal care articles, such as diapers, feminine hygiene products and the like.
The nonwoven fabrics identified above particularly refer to webs made on the spunbond and meltblown processes. For instance, spunbond webs are typically produced by heating a thermoplastic polymeric resin to at least its softening temperature. The polymeric resin is then extruded through a spinnerette to form continuous fibers, which can then be subsequently fed through a fiber draw unit. From the fiber draw unit, the fibers are spread onto a foraminous surface where they are formed into a web and then bonded such as by chemical, thermal, or ultrasonic means.
Meltblown fabrics, on the other hand, have been conventionally made by extruding a thermoplastic polymeric material through a die to form fibers. As the molten polymer filaments exit the die, a high pressure fluid, such as heated air or steam, attenuates the molten polymer filaments to form fine fibers. Surrounding cool air is induced into the hot air stream which cools and solidifies the fibers. The fibers are then randomly deposited onto a foraminous surface to form a web. The web has integrity as made but may be additionally bonded.
In the past, nonwoven webs and films have been made almost exclusively with thermoplastic polymers, such as nylon, polyester, polypropylene and polyethylene. Such polymers, however, can degrade during the melt processing operations used to form the articles. For instance, during the formation of many polymeric articles, the polymers used to make the products are exposed to various harsh conditions which can adversely affect the properties of the polymers. For example, during extrusion, a polymer is not only subjected to various external forces, but is also heated to high temperatures. Due to these conditions, fibers and films made from these polymers can have decreased strength and elasticity, can become brittle, and can yellow or otherwise degrade in color, which can make a product with a short product life and decreased properties.
In the past, in order to correct or minimize the above-described problems, much research has been done to increase the melt stability of the polymers and improve the physical properties and the resiliency of products made from the thermoplastic polymers. Unfortunately, although some improvements have been made to the polymers, the cost of the polymers have increased.
Additionally, as the polymers are thermoplastics without permanent crosslinks, the products readily melt, dissolve, and/or burn. Products made from thermoplastic elastomers without permanent crosslinks also have reduced elastic properties.
In view of the above deficiencies and drawbacks, a need currently exists for a replacement to conventional thermoplastic polymers used to make films, fibers and nonwoven webs. In particular, it would be very desirable if, besides thermoplastic polymers, polymers that form permanent crosslinks were available for forming polymeric articles. Specifically, a need exists for a replacement polymer that has improved properties for some applications.
The present invention recognizes and addresses the foregoing disadvantages and others of prior art constructions and methods.
In general, the present invention is directed to forming polymeric articles from thermoset polymers as opposed to thermoplastic polymers. Of advantage, many thermoset polymers are less expensive than some conventionally used thermoplastic polymers. Further, thermoset polymers can be more flame-resistant, more chemical-resistant, and can have better elastic properties than many thermoplastic polymers.
As used herein, a thermoset polymer refers to resins which change irreversibly under the influence of heat from a fusible and soluble material into one which is infusible and insoluble through the formation of a covalently crosslinked, thermally stable network. In contrast, thermoplastic polymers soften and flow when heat and pressure are applied, the changes being reversible. Examples of thermosetting resins include ureas, phenolics, malamines, urethanes, and epoxys.
In the past, it was not believed possible to readily form thermoset polymers into fibers and films. In particular, it was believed that conventional thermoplastic polymer processing equipment could not be used to make thermoset polymer filaments and films. The present inventor, however, has discovered a method for accomplishing this task.
The above objects and advantages of the present invention are achieved by providing a method of forming polymeric articles from thermosetting polymers. The method includes the steps of providing a thermoset prepolymer composition, which is initially in a fluid state. The prepolymer composition is energy activatable meaning that when the prepolymer composition is treated by a particular energy source, the composition will undergo an irreversible chemical transformation to form a post reaction thermoset polymer.
In accordance with the method of the present invention, the prepolymer composition is extruded through at least one die to form a polymeric article. The polymeric article is then treated by an energy source after the prepolymer composition has exited the die. The energy source causes the prepolymer composition to gel and/or harden and form the post reaction thermoset polymer.
The polymeric articles that can be made according to the present invention include, for instance, films, fibers and nonwoven webs. In one embodiment, the polymeric articles can be deposited upon a foraminous surface as they are being formed and after or during contact with the energy source. As used herein, a foraminous surface refers to a surface upon which the polymeric article is placed after extrusion and can include, for instance, a traveling screen or conveyor.
Examples of thermoset polymers that can be used in the process of the present invention include polyurethanes, silicone polymers, phenolic polymers, amino polymers, epoxy polymers, and the like. In general, any thermoset polymer can be used in the process that can be made from an energy activatable prepolymer composition.
The energy source used to convert the prepolymer composition into the thermoset polymer can vary. For instance, in one embodiment, when the prepolymer composition is heat activatable, the energy source can comprise a heated gas. Other energy sources that may be used in the process of the present invention include ultra sonic sound waves, irradiation, infrared radiation, and microwave energy. In general, the energy source that is used to activate the prepolymer composition will generally depend upon the prepolymer composition used and the catalyst present within the composition if a catalyst is needed.
In one particular embodiment, the present invention is directed to a method of forming a nonwoven web from a thermoset polymer. The method includes the steps of extruding an energy activatable thermoset prepolymer composition through a die to form fibers. The fibers are contacted with an energy source, which causes the prepolymer composition to irreversibly polymerize and form a thermoset polymer. Thereafter, the fibers are formed into a nonwoven web such as, for instance, by being deposited upon a foraminous surface during formation of the fibers.
The chemical prepolymer composition may include a solvent or processing aid to lower the viscosity of the composition for ease of extrusion including higher throughputs and lower temperatures. The solvent could help retard the crosslinking reaction and could partially or totally evaporate during or after fiber and/or film formation.
In order to prevent the prepolymer composition from solidifying within the die, the die can be cooled during the process. For instance, fibers exiting the die can first contact a first gaseous zone prior to contacting the energy source. The first gaseous zone can comprise a stream of cool air which flows around the die and generally in the same direction as the prepolymer composition being fed through the die. This air is maintained at a temperature so the crosslinking reaction is very slow. The fluid composition, either in the form of a filament or film, can thus be attenuated into fine fibers or thin films.
After passing through the first gaseous zone, the fibers can then be treated by an energy source, such as a heated gas which can cause the prepolymer composition to crosslink and form a thermoset polymer. The heated gas can be contained in the surrounding atmosphere or can be contained in a gas stream flowing generally in the same direction as the first gas stream. After contacting the heated gas, the fibers can then be deposited upon a foraminous surface to form the nonwoven web.
In one further alternative embodiment of the present invention, a solid additive can be incorporated into the fibers or film during formation. Additionally, additives can be added that go into the fiber matrix or attach to the film. The solid additive can be pigments, dyes, opacifiers, pulp fibers, other natural or synthetic fibers, or superabsorbent particles. As used herein, a superabsorbent material refers to materials that absorb more than 20 grams of liquid per gram of solid. The solid additive can be combined with the polymeric article being formed during contact with the energy source. For instance, when the energy source is a heated gas stream, the additive can be present within the stream.
Other objects, features and aspects of the present invention will be discussed in greater detail below.